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You pride yourself on keeping your home clean and safe. But the biggest threat to your well-being isn't visible to the naked eye. Pollution in your home is often 2 to 5 times higher than it is outdoors, according to the EPA. "The air in your house contains pollen, mold, and ozone that leach in from the outdoors, as well as pet dander and pollutants from household cleaning products," says Ted Myatt, ScD, a senior scientist at the consulting firm Environmental Health and Engineering, Inc.

Come winter, weatherproofing combined with heated, dry air can boost indoor pollution levels even higher by sealing in airborne toxins and lowering levels of humidity. The combination of the two can pose an even greater risk. "Exposure to indoor pollution is associated with allergies, severe asthma, hospitalizations for cardiovascular and respiratory disease, and even heart attacks," Dr. Myatt says.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Opening up windows when it's freezing outside sounds, well, cold (and costly). But sealing a house too tightly doesn't allow the entry of new oxygen or the escape of carbon dioxide that you exhale. As a result, your body doesn't get the amount of oxygen it needs, and you end up feeling tired and lethargic, explains Matthew Waletzke, a certified building biology consultant in Long Island, NY. He adds, "Oxygen levels can be especially low in a sealed bedroom after a night's sleep."

Clear the air: Open your bedroom windows for 5 to 10 minutes after you wake up in the morning and again before you climb into bed at night; this is enough time to let carbon dioxide out and oxygen in without chilling the rest of your house.

Dragging winter blankets out of the attic and lugging decorations up from the basement stirs up dust, triggering allergy symptoms such as itchy eyes, wheezing, and congestion, says James Sublett, MD, chief of allergy at the University of Louisville School of Medicine.

Clear the air: Take boxes outdoors to wipe off the dust, then wipe them down (along with what's inside) before you bring them back in and plop them in the front hall. Wash any blankets or linens in hot water before you use them (same goes for winter clothes that can go in the washing machine). You can also put on an N95 dust respiratory mask (available at drugstores) before you head to the attic or basement, Dr. Sublett says. It'll shield you from 95% of airborne particles that set off sneezing fits, but you'll probably still want to dust off boxes if you plan on taking off the mask.

Scented candles, especially the industrial strength (and size) that many people light around the holidays, give off more than fragrance—studies show they produce tiny bits of pollution known as particulates that can inflame the respiratory tract and aggravate asthma, Dr. Sublett says. This is especially true if some of the dust you kicked up unearthing Grandma's decorations is still floating around. "Allergens like dust can hitch a ride on particulates, enter deep into your lungs, and make breathing more difficult," he explains.

Clear the air: Stop burning candles, especially the ones inside large jars, which tend to send even more particulates into the air, says Jeffrey May, principal scientist at May Indoor Air Investigations in Tyngsborough, MA, and author of My House Is Killing Me! If the holidays aren't the same for you without that soft, candlelit glow, choose unscented tapered candles, and place them far from vents and other air sources.

Exhaust fans work by sending the stale indoor air outside and replacing it with fresh air. However, running powerful fans such as commercial-size kitchen fans, large exhaust fans, or bathroom fans all at once (especially for an extended period of time) can redirect exhaust gases that may include deadly carbon monoxide fumes produced by gas or oil heaters back into the house instead of up and out the flue, explains Max Sherman, PhD, a senior scientist at Lawrence Berkeley National Laboratory.

Clear the air: Turn exhaust fans off as soon as they've done their job, or consider replacing a manual switch with a timer to limit unnecessary use. Install carbon monoxide detectors as well; they're just as important as fire detectors.

The upside to winter's drier air is that it makes it difficult for mold to grow. But existing mold from damp basements and lingering spores in air-conditioning systems can become airborne (and stay there) if all the windows are closed, May says. Mold can irritate your eyes, cause congestion, and worsen existing respiratory problems.

Clear the air: Change your heating system filters every 3 months, Dr. Sublett says. Filters act like armed guards, holding hostage pollutants that feed mold—such as human skin cells, pollen, and pet dander—so they can't escape into your indoor air. May recommends a filter with a minimum efficiency reporting value (MERV) rating of at least 8. Check the packaging. And have a professional service your heating system annually. Summer may be the best time—that way you can fix problems before you need the heat.